Today, cellular wireless carriers primarily use second-generation cellular systems to provide cellular communications. The most widely used systems are the global system for mobile communications (GSM), which uses a hierarchy of time division multiplexing access (TDMA) frames, and code division multiple access (CDMA), which is a spread spectrum-based technique. In addition, significant work is underway on the development of third-generation wireless communications systems.
Regardless of the technology used, cellular carriers provide wireless services by partitioning their service territories into cells. Ideally, a cell is in the shape of a hexagon; however, in practice, topological limitations and other considerations lead to significant variations in a cell's topology. Each cell may be further partitioned into a few (e.g., 3 or 6) cell sectors. Every cell sector has a Base Transceiver Station (BTS) with multiple transceivers that transmit and receive signals at multiple frequencies. Thus, the BTS serves as a mobile station's access point into the communications network.
Designing and operating a wireless cellular network is quite complex. One approach a carrier typically uses to mange its network is to partition its service territory into small “bins” (also called pixels) and to evaluate the service quality at each bin. For example, a 50-kilometer by 50-kilometer service territory can be partitioned into more than 60,000 bins of 200 meters by 200 meters. Using propagation models and field measurements, a carrier then derives the mean and standard deviation of the signal strength received at every bin from every BTS. The matrix of the expected signal strengths among all bin-BTS combinations is known as the Received Signal Strength Indicator (RSSI) matrix. A carrier uses this matrix to determine if a mobile station in a particular bin is being adequately served. In particular, multiple BTS's may adequately serve a mobile station in a particular bin. Typically (e.g., in GSM), the BTS that transmits the strongest signal to the mobile location serves the mobile station. Alternatively (e.g., in CDMA), multiple BTS's that transmit the strongest signals to the mobile location may serve the mobile station simultaneously. A mobile station in a specific bin is served well if the ratio of the carrier signal strength to that of the sum of the interfering signal strengths is above a certain threshold.
A carrier also continuously collects performance data at every BTS, including data regarding the carried load, blocked calls, dropped calls, quality of connections, etc. From this information, the carrier attempts to infer the service provided to different bins in the territory.
However, carriers today do not know the offered load (represented, for example, in units of bits per second, number of time slots per second, minutes of call per minute, etc.) generated at individual bins at different hours of the day. Knowing each bin's offered load would better facilitate the carrier's planning and operational processes. In particular, knowing the offered load at each bin would allow the carrier to better manage network resources and to provide better service to those bins with higher loads by allowing more intelligent assignment of frequencies among the BTS's and by facilitating load balancing across the BTS's through the fine-tuning of different control parameters.
More specifically, a fundamental problem with many of today's cellular network technologies (e.g., in GSM) is the assignment of frequencies among the BTS's. The total number of available frequencies is limited so that each frequency must be assigned to multiple BTS's. However, the assignment of a given frequency across multiple BTS's must be done so that the mobile stations experience negligible interference. For example, Karen I. Aardal, Stan P. M. Van Hoesel, Arie M. C. A. Koster, Carlo Mannino, and Antonio Sassano present in the paper “Models and Solution Techniques for Frequency Assignment Problems,” that appeared as a report of Zentrum fur Informationstechnik Berlin (ZIB), ZIB Report 01-40, December 2001 a survey on frequency assignment models and algorithms, which paper is hereby incorporated by reference. As the authors describe, the models/algorithms use an aggregation of the bins to cell sectors and use as input a representative expected signal strength received at every BTS from every other BTS. However, frequency assignment models and algorithms would be improved by assigning weights to the bins wherein these weights are set equal to the bin estimated offered loads. In addition, Jean-Marie Bourjolly, Leslie Dejoie, Ke Ding, Oumar Dioume, and Michel Lominy emphasize in their paper “Canadian Telecom Makes the Right Call, Frequency Allocation in Cellular Phone Networks: an OR Success Story,” OR/MS Today, 29, 40–44, April 2002 that the resulting frequency plan should be evaluated at the bin level rather than at the BTS level with the objective of minimizing, for example, the number of bins with unacceptable reception quality. Knowing the bins that generate higher offered loads would allow a carrier to spend more resources and thereby provide better service to bins that generate more demand.
BTS load management is also an important issue in order to avoid uneven congestion and blocking of call attempts. Carriers can, for example, adjust the transmission power from specific BTS's and thus change the area served by each of the BTS's. By reducing a certain BTS's transmission power, some of the bins that this BTS previously served will now receive a stronger signal from other BTS's, which will now serve those bins. Thus, the fine-tuning of the power parameters at a BTS is used to balance loads among the BTS'S. Any load-balancing scheme that takes into consideration the loads generated at each bin would more effectively fine-tune the power parameters.